High end sports racquets are typically made with frames of composite material, such as carbon fiber-reinforced epoxy, which is commonly referred to as "graphite". Such materials have a high stiffness-to-weight ratio, and therefore allow frames to be made very lightweight, for easy maneuverability, and at the same time stiff for good power.
Sports racquets made of composite materials are normally provided with a plastic grommet strip on either side of the frame, and a plastic bumper strip that extends around the outer portion of the racquet head. The strips contain hollow grommet pegs that extend through string holes drilled in the frame. After crossing the stringing area, each string exits the stringing area through one grommet peg and extends along the grommet or bumper strip surface to the next grommet peg, where it re-enters the stringing area. The purpose of the grommet and bumper strips is to protect the strings and composite material from damaging one another, particularly at the tops of the string holes where the strings bend sharply.
In addition to being susceptible to damage from the strings, composite materials can be damaged upon impact with a hard surface. In squash or racquetball, the racquet frame can be damaged upon striking the floor or a wall. In tennis, the frame can be damaged by hitting court surface, which can occur when hitting a low shot.
The games of squash and racquetball require quick reflexes and fast racquet movements. It is therefore desirable to make the racquets lightweight. Also, although lighter weight is not as important in tennis, there currently is a preference among some tennis players for lighter weight racquets. One of the ways of reducing racquet weight is to reduce the wall thickness of the frame. However, susceptibility to damage from impact with the court surface increases as the wall sections are made thinner.
Normally, the outside surface of the racquet head is provided with a stringing groove. The two grommet strips fit into the stringing groove on either side of the racquet. The bumper strip connects the ends of the two grommet strips. The bumper strip has a center strip portion which, like the grommet strips, contains grommet pegs and is disposed in the stringing groove. The bumper strip also has two flanges that extend laterally from the center strip portion so as to cover the racquet frame outer surface in order to protect the head from impact damage. Because damage that is the result of impact with a court surface is most likely to occur in the outer region of the head, and in order to avoid adding unnecessary weight, the bumper strip is normally confined to the outer head region only.
Due to the high stringing tensions typically used in sports racquets, the grommet and bumper strips need to be made of a relatively hard material in order to protect the frame from being damaged by the strings. Thus the grommet and bumper strips are typically made of a nylon having a hardness in the range of Shore D 80 durometer.
U.S. Pat. No. 5,165,687 discloses gluing a pair of strips of viscoelastic damping material to the outer frame surface in order to dampen high frequency vibrations of the frame. In one embodiment, the strips are very thin, i.e., about 1.0 mm, and are positioned between the frame and the flanges of a conventional bumper strip. In an alternative embodiment, the flanges of the bumper strip are removed, in order to utilize a "thick dampening layer" of about 3.0 mm. The patent notes that the "thick damping layer" will provide an additional effect of absorbing ground impact shock better than a plastic bumper guard.
According to the '687 patent, either embodiment will add about 10 grams of additional weight to the outer region of the racquet. In the case of the thin strip embodiment, the density of the viscoelastic materials used in the '687 patent will result in a total weight, using 1 mm thick strips, of 10 gram. The thick dampening layer will of course weigh more, but the additional weight increase is partially offset by the weight savings resulting from removing the two flanges of the bumper strip.
Adding 10 grams of weight to the outer region of the racquet frame is very undesirable, because it makes the racquet less maneuverable. Moreover, while it would be desirable to improve the racquet's ability to absorb ground impact shock, the proposal of the '687 patent to remove the flanges of the bumper strip and glue a thick dampening layer to the frame appears to have certain drawbacks. In particular, when the frame scrapes a court surface, large shear stresses and heat can result, which would tend to tear the glued layer off the frame, as well as damage the relatively soft viscoelastic material. It would thus be desirable to continue to use a hard nylon bumper strip.